The invention relates to improvements in a ground, and possibly thereover, especially a weak ground such as a clay.
Various types of foundations are already known for erecting facilities, buildings, and supermarkets or for obtaining an embankment for a highway, for example.
It is especially known to dispose piles in the ground above which the building is erected. However, it is traditionally required to dispose a network of structural inclusions (beams, piles, etc.) made of a traditional reinforced concrete for supporting the floor of the building, or the road. If the piles are disposed in a mechanically weak ground, the strength induced by the building is typically essentially transmitted by the piles to a harder portion of the ground. Thus, substantially no charge of the building is transmitted to the weak ground.
It is to be noted that, in the present description, a ground is considered as weak if its modulus of elasticity (E), also called Young""s modulus, is less than about 8 to 9 MPa. A series of so-called ballasted columns is sometimes used in such a situation. Typically, such ballasted columns are wells dug into the ground and having a diameter between about 0.5 m and 1.20 m. The wells are filled with stones or a mixture of sand and gravel having dimensions typically comprised between 10 mm and 60 mm.
If the ground is too weak (e.g. the modulus of elasticity of the ground is less than about 5 MPa), such columns often break off, if a quite high building is erected thereon. Embedding rigid inclusions within the ground is another solution. Such rigid inclusions are piles having a small diameter. The distance between two piles is typically between about 1.50 m and 2.50 m. The piles are either micropiles including a metallic tube, or full piles made of a traditional concrete. But the stiffness is then typically too high to share the loads between the soil and the inclusions.
The equivalent stiffness of a network comprising such rigid inclusions is very high. Then, more than 95% of the charge is transmitted to the piles, and only 5% is transmitted to the weak ground.
A ground displacement auger head adapted for making inclusions in a ground is known from WO-A-95/12050 in which the auger head comprises a tip, a displacement body having at least over a lower portion a core diameter increasing in a direction away from said tip, and at least one screw flange extending at least over said lower portion of the displacement body, the auger head being characterised in that the screw flange has a pitch which increases at least over said lower portion of the displacement body in the direction away from the tip.
In WO-A-95/12050, a main object is to present an auger head by which the ground can be displaced more efficiently, requiring less energy during screwing in, and which allows also to screw through more resistive, in particular more sandy, layers of ground.
Another drill is described in EP-B-228 138 in which the object is to avoid that a high resistance be encountered during screwing into the ground. To that end, the tubular screw drill described in EP-B-228 138 comprises a screw fin (also called screw flange) and is characterised in that said screw fin extends between an end of the drill and a displacer which consists successively of a first closed spiral part having the same screw direction as the fin, and a second closed spiral part having the opposite screw direction.
In the present invention, it is considered that the drills of the prior art, and especially the above-mentioned drills, have drawbacks including a too high resistance still encountered during forcing the drill in the ground.
So, an object of the invention relates to a system having an improved efficiency for reinforcing a ground on which is disposed a loading structure, such as a building.
A further object is to dispose in the ground inclusions having a modulus of elasticity (E) substantially equal to the modulus of elasticity of the so-called ballasted columns, but which do not weak off so often.
Another object of the invention is to offer an improved concrete material having a modulus of elasticity (E) less than a traditional hard concrete, so that a structure made of such an improved concrete does not systematically concentrate all the strengths in the inclusions.
A further object relates to an improved drill adapted for digging an annular hole in a ground, and for providing said hole with a concrete or a structural material.
Another object refers to an improved ground displacement auger head adapted for drilling a hole in a structure.
The first object of the invention is achieved through a device for reinforcing a ground on which is disposed a loading structure, such as a building, the device comprising:
a series of structural inclusions disposed essentially vertically within the ground and adapted to mechanically reinforce said ground, the series of inclusions having an external diameter (D) comprised between substantially 0.1 m and 0.8 m (preferably 0.2 m and 0.6 m), and two adjacent inclusions being separated in the ground by a distance (A) such as 2D xe2x89xa6A xe2x89xa610 D, said series of inclusions being made of a material having a Young""s modulus (long term) comprised between 500 and 10,000 MPa, and preferably between 1,000 and 10,000 MPa,
a load transmitting layer interposed between the ground and the loading structure disposed thereon, for transmitting and distributing the load from the loading structure to both the ground and the series of inclusions, the load transmitting layer having:
a Young""s modulus less than the Young""s modulus of the series of inclusions (viz. of every inclusion of said series),
a first nominal height between two adjacent inclusions of said series of inclusions, and
a second height less than the first nominal height at locations situated just above the series of inclusions, where the load transmitting layer is plastically compressed by the series of inclusions which partially penetrate within said load transmitting layer.
In the instant description and claims, it is considered that:
D is indifferently either an external diameter of one inclusion of the series of inclusions, or the external diameter of the cylinder having the same volume as one inclusion of said series,
A is indifferently either the horizontal distance between two adjacent inclusions of the series, or the length of one side of a square equivalent to said distance between two adjacent inclusions in the ground,
10 bars=1 MPa
all the values in the present text are given to within 10%,
xe2x80x9cModulusxe2x80x9d are long term modulus (a few days or weeks, at least) as opposed to immediate modulus calculated immediately after the load has been applied to the corresponding structure,
a (structural) inclusion is an essentially vertical, elongated structure adapted to mechanically reinforce a ground, especially a weak ground. An inclusion can be made of different types of materials, such as aggregates of stones, mortar, specific concrete (especially concrete having a low modulus of elasticity, as above-mentioned). The inclusions can be annular tubes or parallelepiped piles. Their section can be rounded or parallelepiped. If the (external) section is rounded, it is considered that the corresponding inclusions have an (external) diameter. The inclusions can be either hollow (such as annular piles) or full of material.
According to another feature of the invention, the device for reinforcing the ground comprises:
a series of structural inclusions disposed within the ground and adapted to mechanically reinforce said ground,
a load transmitting layer interposed between the ground and the loading structure disposed thereon, for transmitting and distributing the load from the loading structure to both the ground and the series of inclusions, the load transmitting layer having a Young""s modulus less than the Young""s modulus of the series of inclusions, wherein:
(Ei) being the Young""s modulus of (one of) the inclusions, and
(Ec) being the Young""s modulus of the load transmitting layer:
if Dxe2x89xa60.3 m, then 5,000 MPaxe2x89xa6Eixe2x89xa612,000 MPa,
if Dxe2x89xa60.3 m, then 500 MPaxe2x89xa6Eixe2x89xa610,000 MPa, and Ecxe2x89xa680 MPa.
In accordance with the aspect of the invention relating to an improved concrete adapted to be used for manufacturing the so-called xe2x80x9cinclusionsxe2x80x9d, it is proposed that such a concrete consists of an aggregate comprising, in dry weight and for obtaining 1 m3 of aggregate:
between substantially 100 kg and 400 kg of cement and/or lime,
between substantially 10 kg and 50 kg of an elastic substance,
between substantially 500 kg and 1,800 kg of flying ashes and/or sand.
With such a dry powder, it should be possible to obtain a concrete having a long term modulus of elasticity (E) comprised between about 1000 to 2500 MPa, for an ultimate compression strength comprised between substantially 2 and 12 MPa.
Traditionally, the (long-term) modulus of elasticity (E) and the strength "sgr"j (both in MPa) of a concrete are estimated as follows:
E=xc2xdxc3x9711.000 ("sgr"j)⅓ ("sgr"j is the strength of the concrete, j days after having made it).
Preferably, the abovementioned elastic substance is obtained from a powder of rubber, and the granulometry of said substance is preferably comprised between about 0.08 mm and 2.5 mm, while preferably including between about 30% to 70% of grains (or granulars) comprised between 0.5 mm and 1.8 mm, and about 70% to 30% of grains comprised between 0.15 mm and 0.5 mm. Of course, a mixing agent (typically water) will be used to make the mixture and obtain a ready-to be used concrete.
Another object of the invention is an improved xe2x80x9cinclusionxe2x80x9d, such as a pile, adapted to be disposed especially in a weak ground and made of a semi-rigid material, so that:
100 Esxe2x89xa6Eyxe2x89xa61000 Es, 
with (Ey) being the Young""s modulus of both the load transmitting layer and the series of (all the) inclusions, and,
with (Es) being the Young""s modulus of the ground to be reinforced.
The invention also refers to a method for disposing the inclusions of the above series in the ground to be reinforced, as follows:
a drill is screwed in the ground for lowering it in said ground and digging in the ground a shape corresponding to the shape of an individual inclusion, while pushing the ground material forced (or pressed back) by the drill, into the surrounding ground,
then, the drill is unscrewed, or pulled out while still screwed, for bringing it up to the ground surface again, and the space in the ground, released by unscrewing, or extracting, the drill and forcing the ground material, is provided with the material in which the inclusion is to be made of, through the drill.
According to a further feature, the drill is preferably hollow for obtaining a series of annular inclusions.
According to the invention, a drill adapted for digging an annular hole in a ground, and for providing said hole with the material in which the inclusion is to be made of, preferably comprises:
a central tube having an axis, a top end portion and a bottom end portion,
a first helical blade axially arranged around the top end portion of the central tube,
a second helical blade axially arranged around the bottom end portion of the central tube,
the second helical blade having a diameter larger than the first helical blade,
a lining partially arranged around the first helical blade, and extending axially beyond said first helical blade, the lining comprising a double-walled portion defining an inner chamber in fluid communication with a duct crossing the first helical blade, said duct being in fluid communication with the central tube, for injecting therein the concrete,
a third helical blade arranged around the lining,
a movable trap closing an opening of said inner chamber of the lining, said trap being adapted to be opened for allowing the concrete to fill the annular hole.
As above-mentioned, it is a further object of the invention to improve the penetration of a drill within the ground, said drill being notably adapted to be used for xe2x80x9ccreatingxe2x80x9d the so-called xe2x80x9cstructural inclusionsxe2x80x9d (which can be as hollow-viz. annular as solid-viz. completely full of material). It is further intended to get a so efficient compression of the ground when screwing the drill that the pulling out thereof is especially easy and no inverted upper screwing flange at the upper section of the drill is required. Furthermore, unscrewing is useless when pulling out the drill.
Another object is to reduce the torque required for moving the drill within the ground and to enable said drill to more easily penetrate through a highly (mechanically) resistive ground. To that end, it is proposed a drill comprising a core having:
an axis,
an end provided with a front drilling head adapted to be forced in the soil at the first, and
a convex external lateral surface from which a series of blades project, radial to the axis, the blades having a length substantially parallel to the axis.
A complementary object of the invention is to optimise both the penetration of the drill and the lateral compression of the ground (ground displacement). To that end, the blades of the drill of the invention are preferably helically wound in an helix round the axis of the core and individually have a length parallel to the core axis.
A further object of the invention is to improve the progressive destructuration of the ground in proportion as the drill is pushed in the ground. To that end, the blades of the drill of the invention preferably project from said convex external lateral surface of the core on a width which increases as the axial distance of a determined blade of said series and the front drilling head increases.
Another object of the invention is to provide blades which improve the above-mentioned effect of destructuring the ground, while being easy to manufacture, at a low cost. To that end, at least some of the blades have a step for having their width varied, so that said blades individually have a lower section having a width lower than the width of a higher section, said lower section being axially located closer from the front drilling head than the higher section.
For optimising the effect of destructuring the ground, the width of each section of the blades section increases from one blade of said series to another blade, as the axial distance between a determined blade of said series and the front drilling head increases. During the drilling operation, the drill has preferably to be precisely screwed in the ground. For having the drill precisely directed in the ground:
in the immediate vicinity of the front drilling head of the drill according to the invention, the core is provided, on its convex external lateral surface, with at least one screw flange wound round the core axis, and
said at least one screw flange axially extends along the core on a predetermined distance (for example comprised between half-a-turn and two turns), so that the blades are axially located apart from the front drilling head, the screw flange(s) being axially interposed between the front drilling head and said series of blades. So, at the lower section of the drill, the blades are protected and, at the lower end (screw flange section), the ground is only destructured before being preferably decompressed, and then progressively laterally displaced by the blades.
A further object of the invention is to have the global costs reduced with no alteration of the performance of the drill. To that end, one of the above-mentioned screw flange(s) can extend round the core on a longer distance (preferably at least three or four turns), the series of blades of the drill extending then parallel to the core axis, between a first and a second sections of the screw flange (i.e. between two adjacent turns thereof) So, a tubular screw drill according to the prior art, having at least one screw fin, or screw flange, can be basically used, while being provided with said series of ground destructuring blades of the invention.
Another object of the invention is to optimise the lateral displacement of the ground when screwing the drill. To that end, the blades preferably begin, at a lower end thereof, at an axial distance from an upper end of said at least one screw flange, so that an open space is interposed between the blades and said at least one screw flange. Said open space allows the ground to be destructured by the lower screw flange(s) to be decompressed before being laterally displaced (or compacted) by the blades. It is to be understood that said open space is an intermediate zone located round the external surface of the core free of any blades or screw flange.
A further object of the invention is to provide an efficient obturator at the lower end of the drill, near the drill head. Typically, such a drill comprises an axial bore provided through the tubular core thereof, said axial bore having a lower, opened end. Often, in the drills of the prior art, the obturators provided for opening or closing said lower, opened end comprise either a drilling tip or a simple plate which are abandoned at the bottom of the drilling, when the drill is finally pulled upwardly, and the concrete (or any other material, such as a mortar) is injected into the drilled hole. Such solutions are expensive and providing the drill with such an obturator each time the operator has to screw the drill in the ground incurs a loss of time. For avoiding such drawbacks, the drill of the invention preferably comprises an axially movable skid permanently attached to the core and located at the lower end thereof, the movable skid preferably comprising an angulated, splayed out wall adapted for engaging an angulated splayed out wall located at the lower, opened end of the axial bore, so that a valve is defined at said end, and a torque can be efficiently transmitted to the skid (when it engages the core) Furthermore, the movable skid is preferably pendularly attached to the tubular core, and has a front ground deflector. Such a pendular skid gives the operator an indication of how the drill is positioned in the ground. Further, the skid is a self-direct closing system and the drill is pushed into the ground. The skid is self-centering. Furthermore, if the splayed out walls of the skid and the tubular core, respectively are V-shaped, the hollow section at the lower, opened end of the bore for allowing the concrete, or mortar is maximum while the deflection of said flow is minimum.